This invention relates to electronic musical instrument tone generation. In particular the invention deals with the problem of simultaneously synthesizing the many different tones of a pipe organ electronically. Further, the invention deals with the problem of creating a plurality of simultaneously sounding, aesthetically desirable tones at reasonable cost.
Because of the long history and continued popularity of the pipe organ, high-quality electronic organ designers endeavor to understand and emulate the sounds of the pipe organ so as to retain, or even enhance, the particular characteristics which make the pipe organ so aesthetically appealing. At the same time the designers seek to exploit the unique advantages of the electronics approach to organ design.
One of the most distinctive features of the pipe organ is that it consists of a large collection of essentially independent tone generators, viz. the pipes. The fact that the pipes are spatially separated and have slightly different frequency and speech characteristics gives the pipe organ a dramatic ensemble sound.
One of the challenges of the electronic organ designer is to emulate this ensemble effect. The most direct method would be to replace each pipe with a separate, complete electro-acoustic pipe synthesizer, including tone generator, audio power amplifier, and speaker. Obviously this would be economically and perhaps even physically impractical. Fortunately, it has been discovered over the years that various liberties can be taken in designing the electronic organ so as to reduce the physical size and cost while still retaining much of the aesthetically appealing and desirable characteristics of the pipe organ. For example, it is common practice to combine the tones simulating many different pipes into a composite signal which is then amplified and converted to sound through an electro-acoustic speaker. Hence, many tones may share a single audio channel, i.e. audio amplifier and speaker, resulting in physical size reduction and cost savings. The selection of tones which are to be combined into one of several audio channels is one of the high-quality organ designer's principal challenges. The objective is to simulate the spatial separation of the individual pipes of a pipe organ by having the various electronically synthesized pipe tones emanate from some "reasonable" number of spatially separated speakers. This combining of plural tones into a limited number of audio channels has been done since the beginning of electrical and electronic organ technology. However, the development of the digital organ has permitted the combining of tones into composite signals in several audio channels to be accomplished with unprecedented efficiency. Furthermore, subsequent to its inception, many improvements have been made to the digital organ resulting in even more successful synthesis of the pipe organ.
With the invention of the digital organ came a degree of control previously unattainable. In the basic form of this type of organ, digital representations of organ pipe tone waveshapes and/or combinations of such waveshapes are stored in a memory. Activated keys on the organ are assigned to a small number of general purpose tone generators. Digital numbers are used to precisely control the repetitive readout from memory of the selected waveshape(s), or combination of waveshape(s), at the musical frequencies corresponding to the activated keys. The resulting digital data is converted to an analog signal by a digital-to-analog converter to form a composite audio signal representative of the keys being played and the tones or pipe voices selected by the performer. For a further description of the basic operation of a digital organ one can refer to U.S. Pat. No. 3,515,792 (Deutsch) and U.S. Pat. Nos. 3,610,799 and 3,639,913 (Watson), which patents are incorporated herein by reference as may be necessary or required for a full and complete understanding of the technology involved in digital electronic synthesis of pipe organ sounds.
The basic digital organ is particularly well suited to combining tones into a single audio channel. The combining is advatageously done in the waveshape memory circuitry. In other words, to combine two different tone waveshapes, it is merely necessary to read out a waveshape which represents the sum of the two selected tone waveshapes.
The basic digital organ is also adept at allowing tonal change from one region of a keyboard to another, as is desired in the synthesis of many voices of the pipe organ, especially certain Mixture voices. This is accomplished by merely addressing different sections of the waveshape memory according to the keyboard region in which the depressed key is located. In this way the particular waveshapes associated with each keyboard region are addressed and read out only by keys contained in the regions of the keyboard respectively associated with the separate sections of the waveshape memory.
Another desired characteristic of pipe organs is the frequency independence or frequency separation among the pipes speaking at the same pitch. This frequency separation adds to the ensemble or chorus effect. It has been observed that this effect can be well simulated by frequency-separating the various audio channels in the organ. In other words, each of the waveshape memory address generators, which are respectively associated with each audio channel, is made to "run" at a slightly different frequency compared to the other generators. One technique for doing this is further explained in U.S. Pat. No. 3,978,755 (Woron) which is incorporated herein by reference as may be necessary or required for a full and complete understanding of the technology involved in separating the frequency of electronically synthesized pipe organ sounds.
Thus, two important characteristics of the pipe organ, spatial separation and frequency separation, can be efficiently simulated electronically using digital electronic organ technology. There are yet, however, other characteristics of pipe organs which must be understood and effectively simulated in order to more closely replicate authentic pipe organ sounds.
It has long been known that the tones produced by many acoustic instruments, such as pipe organs, are not exactly periodic but are quasi-periodic. This is especially true during the attack portion of the tone, although quasi-periodicity is also often found during the sustained (or steady-state) portion of the tone. The term "quasi-periodicity" is used here to describe the deviation from periodicity often observed in these tones. These tones obviously possess a degree of periodicity because the ear perceives these tones as having a specific musical pitch. Musical pitch is associated closely with the concept of periodicity. However, a true periodic signal is one that exhibits exact cyclic repetition at regular intervals as time progresses; the shortest repeating pattern being termed a cycle of the periodic signal and the time interval occupied by one such cycle being termed the period of the signal. If a recording of an organ pipe is analyzed for periodicity, none can be found in the strict sense of the term. Some sections of the recording, particularly in the "steady-state" portion of the tone, do appear to be periodic at first glance; however, closer examination reveals that no two apparent "cycles" of the signal are configured exactly alike. Thus, the organ tone is close to being periodic, but there is a deviation from exact periodicity. This deviation typically is much greater in the attack transient portion of the tone as compared to the "steady-state" portion. It is this deviation from exact periodicity which enriches the tone aesthetically and contributes significantly to the overall favorable perception of its timbre.
The basic digital organ, as described in the Deutsch and Watson patents identified above, is highly adept at generating essentially periodic tones. Inducing the basic digital organ to simulate the various manifestations of the quasi-periodic nature of a pipe organ has been done in several ways. Building upon the insight developed in the pre-digital organ days concerning these various quasi-periodic effects, such as is explained in U.S. Pat. No. 2,989,886 (Markowitz), digital organ designers discovered various ways to produce similar effects in a digital organ.
In further explanation of this effect, U.S. Pat. No. 3,740,450 (Deutsch) discloses a method for simulating a "chiff" sound in a digital organ by combining a stored "chiff" waveshape with the steady-state waveshape during the attack portion of the tone generation. U.S. Pat. No. 4,184,403 (Whitefield) discloses an improved method for generating a time-dependent, variable waveshape, transient sound in a digital organ, which includes the "chiff" effect.
An improvement to the earlier Whitefield patent may be found in U.S. Pat. No. 4,352,312 (Whitefield/Woron) which discloses a method and apparatus for smoothly interpolating between the sequencially read out, stored waveshapes described in the '403 Whitefield patent. U.S. Pat. No. 4,189,970 (Woron) discloses a method for simulating "chiff" in a digital organ by distorting, or modulating, the steady state waveshape during the attack. The resulting transient sound is rich in harmonics because of the modulation of the steady-state tone signal by a segmentation signal.
These patents of Deutsch, Whitefield and Woron are especially suitable for simulating the "chiff" sound of a pipe organ. This "chiff" sound is defined as and generally refers to the initial turn-on transient characteristic of the pipes. Reference can be made to these patents, which are incorporated herein by reference, as may be necessary or required for a full and complete understanding of the "chiff" sound and the technology involved in electronically synthesizing such pipe organ sound.
Another quasi-periodic sound, not limited to the initial turn-on transient time frame, is the low level sound associated with the air flow through the pipe. Reference can be made to the '886 Markowitz patent for a further explanation of this air flow characteristic. The air flow sound adds a subtle randomly varying quality to the overall pipe tone. One method for simulating this pipe characteristic is to utilize the method for creating frequency modulation in a digital organ as disclosed in U.S. Pat. No. 3,794,748 (Deutsch) in conjunction with a randomly varying modulation signal. By judicious choice of variables, a randomly moving quality can be induced into the otherwise periodic signals so as to suggest the air flow effect found in air-driven organ pipes.
The methods discussed in the Deutsch, Whitefield and Woron patents identified above are all useful methods to induce a quasi-periodic action to take place in a basically periodic pipe tone waveshape organ system, i.e. the basic digital organ. However, there is a limited amount of control afforded by these methods. A highly discriminating listener can perceive differences between the pipe organ and its digital organ counterpart. These differences are related to the limited degree of accuracy achieved in simulating the quasi-periodic quality of the actual pipe organ sound by the methods so far discussed. This is due to the fact that these methods utilize the basic digital organ as a starting point. The problem stems from the fact that, in the basic digital organ, only enough information is stored to generate one cycle (or a small number of cycles) of the waveshape to be replicated at the appropriate pitch for audible reproduction. This places certain restrictions on the generated signals in that only certain harmonically related overtones can be reproduced with high accuracy. It is well known in signal analysis theory that periodic signals have spectra consisting only of purely harmonic overtones. It is believed that actual pipe organs generate tones which exhibit non-periodic overtones, at least during the turn-on transient phase. Thus, the basic digital organ as described above cannot be manipulated in any known way so as to perfectly simulate the subtle quasi-periodic aspects of actual organ pipes.
U S. Pat. No. 4,383,462 (Nagai/Okamoto) introduced a method for faithfully reproducing the actual waveshape of a desired tone during the attack transient and decay transient. This was accomplished by storing the complete transient portion of the desired tone in the memory of a tone generator and reading it out upon depression of a key. The decay transient portion of the tone can be reproduced similarly by storing the decay transient in the memory of another tone generator which is read out upon key release. The steady-state is generated using yet another generator of the periodic type described above. Thus, the Nagai/Okamoto technique provides one method for achieving greater accuracy in tone generation, with quasi-periodicity during the attack and decay transient portions of the tone. However, the steady-state, or sustained portion, of the tone suffers from the same limitations as with the basic periodic generator discussed above. This is due to the fact that Nagai and Okamoto utilize a separate periodic generator to simulate the steady-state portion of the tone. The Nagai/Okamoto method is also inefficient in that the technique requires individual tone generators for each portion of the tone.
With the development of the methods disclosed in U.S. Pat. No. 4,502,361 (Viitanen/Whitefield) came the ability to more accurately and more efficiently simulate the pipe organ, including quasi-periodicity during the steady-state portion of the tone. In this type of digital electronic organ, the complete attack transient portion of an organ pipe waveshape is stored in a memory along with a predetermined number of cycles of the "steady-state" sound. For example, an initial portion of the sound of an actual organ pipe may be sampled and the resulting signal placed in the memory of a tone generator. This signal is then read out, upon depression of a key, at the pitch or frequency associated with that key. Because the generated signal is a faithful playback of the originally recorded tone (except for frequency), all the nuances and characteristics of the organ pipe are contained in the generated signal including those related to quasi-periodicity.
A novel feature of this method is a provision to recirculate through a predetermined portion of the stored waveshape data after reaching a designated point in the stored data. Typically, when a key is depressed, the attack transient portion of the recorded organ pipe waveshape is read out along with the predetermined amount of the "steady-state" sound. When this process is completed, the recorded data is, in a sense, "used up" or depleted. At this point recirculation begins, utilizing the same recorded data in order to continue generating the "steady-state" portion of the tone. The Viitanen/Whitefield method is considered to be an improvement over the Nagai/Okamoto system in that only a single tone generator is required compared to the at least two dedicated tone generators in Nagai/Okamoto. Also, the method of Viitanen/Whitefield provides for quasi-periodicity during the "steady-state" portion of the tone.
As previously stated, the "steady-state" portion of acoustically produced tones is often enriched by quasi-periodic qualities. It has been determined that the quasi-periodicity occurring during the "steady-state" portion of the tone does not require the degree of exactness required during the attack transient portion. Moreover, the discriminating ear is more conscious of the details of the sound during the attack transient portion of the tone generation and less concerned with the subtle quasi-periodic details during the "steady-state" portion of the sound. Thus, exact read out during the attack, and recirculation during the "steady-state" as described in the Viitanen/Whitefield patent produces excellent results in the quest for methods to generate aesthetically desirable organ tones electronically. While the method of Viitanen/Whitefield is not limited to organ tones, it is particularly well suited to generating the sounds of a pipe organ which is the principal problem addressed by the present invention.
One drawback to using the Viitanen/Whitefield system for building an electronic musical instrument capable of generating a plurality of simultaneously sounding, aesthetically desirable tones, such as high quality organ sounds, is cost. The reason for this is the extensive amount of memory required. Such a system is particularly memory intensive when different tones are required for different regions of the keyboard. Another costly aspect of using the Viitanen/Whitefield system for organ construction is the fact that the recirculation logic associated with tones of different pitch cannot be shared. This is because the recirculation logic is an extension of the frequency (or pitch) generator. Even tones of the same pitch often cannot share the same recirculation logic for two reasons. Firstly, frequency separation requires that separate frequency generators, and therefore separate recirculation logic, be used for tones having separate frequencies. Moreover, it is desirable to frequency-separate tones of the same pitch. Secondly, even in the case of tones having the same pitch and no frequency separation, it is often tonally desirable to provide each tone with its own independent recirculation pattern.
In summary, we have discussed two approaches to building an electronic instrument capable of generating a plurality of simultaneously sounding, aesthetically desirable tones. The first approach utilizes the basic digital organ which is geared to generating essentially periodic tones. Aesthetically desirable quasi-periodicity can by induced into the basic digital organ but there are fundamental characteristics, viz. strong periodicity, which limit the degree of exactness in attaining the desired sounds. The second approach utilizes an advanced digital organ concept which removes the limit of the first approach but is relatively costly. Therefore, prior to the discovery of the present invention, there was no known method to generate a plurality of simultaneously sounding, aesthetically desirable tones in a cost effective manner.
It is, therefore, an object of the present invention to permit the generation of a plurality of simultaneously sounding, aesthetically desirable organ pipe and other tones more accurately.
It is also an object of the present invention to reduce the number of memory and logic circuits, and the associated cost, to accomplish the replication of quasi-periodicity in a plurality of simultaneously sounding, aesthetically desirable organ pipe or other tones.
Other objects will appear hereinafter.